The long-term reliability and safety of implantable cardiac leads is critical to the function of implanted medical devices. Conversely, lead anomalies constitute a major cause of morbidity. Representative examples of such medical devices include, but are not limited to, pacemakers, vagal nerve stimulators, pain stimulators, neurostimulators, and implantable cardioverter defibrillators (ICDs). For example, early diagnosis of ICD lead anomalies is important to reduce morbidity and/or mortality from loss of pacing, inappropriate ICD shocks, and/or ineffective shock or pacing treatment of ventricular tachycardia or ventricular fibrillation. Early diagnosis of anomalies in implantable cardiac leads is critical to improving reliability of ICD therapies.
Multilumen ICD defibrillation electrodes or leads include one or more high-voltage conductors and one or more pace-sense conductors. The leads can be implanted as subcutaneous, epicardial, or intravascular leads. Clinically, the most important lead failures have occurred in transvenous right ventricular (RV) defibrillation leads. These leads comprise a distal tip electrode with a fixation mechanism that anchors the lead to the RV myocardium, proximal terminals that connect to the generator, and a lead body connecting the two. The “multilumen” lead body consists of a flexible, insulating cylinder with three to six parallel, longitudinal lumens through which conductors run from the proximal terminals to small pace-sense electrodes and larger shock coil electrodes. RV defibrillation leads have a distal shock coil in the RV. The vast majority of presently implanted transvenous ICD systems deliver therapeutic shocks between the RV shock coil with one polarity during the shock and the housing (“CAN”) of the generator (“active” or “hot” CAN), which has opposite polarity. Defibrillation leads may have either one or two shock coils and one or two dedicated sensing electrodes.
Dual coil vs. single coil leads: Dual-coil leads have an additional proximal shock coil, which usually lies in the superior vena cava (SVC). Dual-coil leads usually deliver shocks with the SVC shock coil electrically linked to the CAN and opposite in polarity to the RV shock coil. Alternatively, shocks may be delivered solely between the RV and SVC shock coils, without using the CAN as a shock electrode.
Integrated vs. true bipolar lead sensing configurations: Integrated-bipolar leads have a single sensing electrode on the tip. They sense the “integrated-bipolar” signal between the tip electrode and RV coil. True bipolar leads have an additional sensing-ring electrode. Their sensing configuration can either be “true bipolar” between the tip electrode and a ring electrode or “integrated-bipolar.”
Insulation breaches have been known to result in a functional failure of conductors within the lead or interactions among said conductors. Functional failure of a pace-sense conductor may result in symptoms caused by loss of pacing functions for bradycardia, cardiac resynchronization, or antitachycardia pacing. Functional failure of a high-voltage conductor may result in fatal failure of cardioversion or defibrillation. In addition, conductor interactions involving pace-sense conductors may result in oversensing leading to inappropriate shocks or failure to pace. Interactions involving high-voltage electrodes may result in shorting the shock output, preventing life saving therapy from reaching the patient and potentially damaging the pulse generator irrevocably.
Thus, one major goal is high sensitivity of diagnosis for the identification of lead failures at the subclinical stage, before they present as a clinical problem. A second major goal is high specificity because a false positive provisional clinical diagnosis of lead failure may trigger patient anxiety and lead to potentially avoidable diagnostic testing. A false positive clinical diagnosis of lead failure may result in unnecessary lead replacement, with corresponding expense and surgical risk.
Insulation breaches or conductor fractures occur most commonly at two regions along the course of a defibrillation lead. The first region is within the pocket, caused either by abrasion of the lead insulation by pressure from the housing (“CAN”) of the pulse generator (lead-CAN abrasion) or twisting and rubbing of the lead within the pocket against other elements of the same or a different lead (lead-lead abrasion). The second region is the intracardiac region between or under the shock coils in a dual-coil lead or proximal to the shock coil in a single coil lead. The second region is a common site of insulation breach for leads in the St. Jude Riata® family, for example, which is subject to “inside-out” insulation breach due to motion of the internal cables relative to the outer insulation. Multiple potential interactions are possible, including, inside-out abrasion of the cable to the RV shock coil against the proximal (SVC) shock coil, resulting in a short circuit within the lead. The lead may also be damaged between the clavicle and first rib, where the lead is subject to “clavicular crush,” usually resulting in conductor fracture.
What is needed is a method and apparatus that focuses on detection of in-pocket lead problems but where the ideas can be extended to problems in other locations along the lead.
Insulation breaches of ICD defibrillation leads within the pocket can result in abrasion of the insulation around any of the cable conductors including the conductor to the RV coil, RV sensing ring, or SVC coil. One of the most dangerous conditions is abrasion of the insulation around the conductor of the RV coil (coil-CAN abrasion). This abrasion results in a short circuit between the CAN electrode and the right ventricular (RV) coil, which prevents defibrillation current from reaching the heart in the event of life threatening ventricular tachycardia or ventricular fibrillation. If the shock is delivered, extremely high current flowing through the shorted output circuit of the ICD may irrevocably damage the generator's components. (Hauser R G, McGriff D, Retel L K. Riata implantable cardioverter-defibrillator lead failure: analysis of explanted leads with a unique insulation defect. Heart Rhythm. 2012; 9:742-749; Hauser R G, Abdelhadi R H, McGriff D M, Kallinen Retel L. Failure of a novel silicone-polyurethane copolymer (Optim) to prevent implantable cardioverter-defibrillator lead insulation abrasions. Europace. 2013; 15:278-283.)
Many ICDs contain circuits that protect the electrical integrity of the generator against shorted high voltage outputs. These circuits abort the shock if the current in the output circuit is sufficiently high, indicative of a short circuit diverting current from the heart. Although such protective circuitry may prevent damage to the generator, the potentially lifesaving shock does not reach the patient. U.S. Pat. No. 7,747,320 to Kroll teaches a backup defibrillation mode method which excludes shorted electrodes during a shock. However, this method applies only during shock delivery of a high output shock in response to detection of ventricular fibrillation or tachycardia by the ICD. Further, such a high-output shock still may have enough energy to ablate additional insulation which will exacerbate the insulation breach and potentially even “spot weld” the exposed conductor to the housing, exacerbating the short circuit. Further, this method cannot be used with single coil leads and it result in shock delivery through only part of the intended defibrillation pathway, with unknown defibrillation efficacy.
Existing technology for diagnosis of lead anomalies in an ICD lead is believed to have significant limitations and shortcomings, especially with regard to diagnosis of high-voltage insulation breaches prior to shock delivery. ICDs routinely deliver low voltage pulses, on the order of about 1.0 volts to about 15.0 volts, or switched AC pulse trains to measure the impedance of the high voltage shock pathway. However, these low-voltage measurements of shock-electrode impedance may not identify insulation breaches in which the insulation's dielectric properties remain intact at low voltages but break down during high-voltage shocks. Clinical case reports indicate that high-voltage insulation breaches may not be detected by these low voltage measurements, and, despite nominal values of such measurements, high voltage clinical shocks have short circuited, preventing the current from reaching the heart and defibrillating ventricular fibrillation. (Shah P, Singh G, Chandra S, Schuger C D. Failure to deliver therapy by a Riata lead with internal wire externalization and normal electrical parameters during routine interrogation. J Cardiovasc Electrophysiol. 2013; 24:94-96.)
U.S. patent application Ser. No. 13/843,145 of Swerdlow and Kroll, filed Mar. 15, 2013 seeks to overcome the limitations of low-voltage pulses for measuring shock impedance by delivering high-voltage, extremely-short (“sliver”) test pulses. However, it may not be practical to deliver such pulses on a routine, daily basis because of the battery power required. Further, if these pulses cause patient discomfort, their delivery may be restricted.
Existing technology for diagnosis of anomalies in pacemaker leads and low voltage lead components is also believed to have significant limitations and shortcomings, especially with regard to early diagnosis. The primary method in the prior art for monitoring pacemaker lead integrity is periodic measurement of electrical resistance, commonly referred to as “impedance monitoring.” Impedance monitoring uses single pulses. Various methods are well-known in the art. These methods provide a value of impedance close to the direct-current resistance.
In the circuit being measured, most of the resistance is at the electrode-tissue interface of the high-resistance tip electrode, and variations of up to 10% in this value are common. Each individual pace-sense conductor (for example, the conductor to the tip electrode or the ring electrode) contributes less than 10% to the measured resistance. Thus even if the resistance in a single conductor doubled or tripled, the overall measured resistance will remain within the expected range. Measurements indicate that resistance does not exceed the expected range until the conductor has lost most of its structural integrity. Thus, resistance measurements are insensitive to partial loss of conductor integrity. Further, resistance measurements have limited specificity. A single, out-of-range value may be an artifact, and marked increases can occur at the electrode-myocardial interface.
Hafelinger et al. (U.S. Pat. No. 5,003,975) and Cinbis et al. (U.S. Pat. No. 5,897,577) summarize some of these methods, which include measurements made directly using either a single pacing pulse or a single independent pulse used only for measuring resistance. McVenes et al. (U.S. Pat. No. 5,741,311) describe use of a longer burst of alternating current at a single frequency. The purpose of these longer (about 100 ms) pulses is to drive the system to a steady-state condition that is not achieved by single, short (less than 1 ms) pacing pulses. Schuelke et al. (U.S. Pat. No. 5,755,742) describe a method for measuring resistance of defibrillation electrodes by applying a test voltage applied to a different excitation current pathway. Kroll et al. (U.S. Pat. No. 5,944,746) described an automated method for periodic measurement of the resistance of the high-voltage (defibrillating) coil in ICD electrodes. Gunderson et al. (U.S. Pat. No. 7,047,083) described a method and system for automated, periodic, measurements of resistance in conductors attached to an ICD or pacemaker. However, these methods identify lead anomalies before inappropriate shocks in only about a third of ICD patients who have conductor fractures and an even lower fraction with insulation breaches. (Swerdlow C D, Gunderson B D, Ousdigian K T, Abeyratne A, Sachanandani H, Ellenbogen K A. Downloadable software algorithm reduces inappropriate shocks caused by implantable cardioverter-defibrillator lead fractures: a prospective study. Circulation. 2010; 122:1449-1455) (Sung R K, Massie B M, Varosy P D, Moore H, Rumsfeld J, Lee B K, Keung E. Long-term electrical survival analysis of Riata and Riata ST silicone leads: National Veterans Affairs experience. Heart Rhythm. 2012; 9:1954-1961) (Ellenbogen K A, Gunderson B D, Stromberg K D, Swerdlow C D. Performance of Lead Integrity Alert to assist in the clinical diagnosis of implantable cardioverter defibrillator lead failures: analysis of different implantable cardioverter defibrillator leads. Circ Arrhythm Electrophysiol. 2013; 6:1169-1177.)
A different method for monitoring defibrillation lead sensing integrity is based on sensing of rapid nonphysiological signals associated with lead conductor fractures by the ICD pulse generator. Repetitive oversensing of nonphysiologically-short intervals may indicate lead conductor fracture even if lead resistance is normal. Gunderson et al. (U.S. Pat. No. 7,289,851) described a Lead-Integrity Alert that incorporates both ICD-based measures of oversensing based on the nonphysiologically-rapid rate of sensed signals and periodic measurements of resistance. This method, combined with automatic ICD reprogramming, improves warning time before inappropriate shocks caused by lead-related oversensing. Nevertheless, approximately 40% of patients receive inappropriate shocks with conductor fracture (Swerdlow C D, Gunderson B D, Ousdigian K T, Abeyratne A, Sachanandani H, Ellenbogen K A. Downloadable software algorithm reduces inappropriate shocks caused by implantable cardioverter-defibrillator lead fractures: a prospective study. Circulation. 2010; 122:1449-1455).
In addition to limited sensitivity, present methods for diagnosing lead anomalies have limited specificity resulting in false positive diagnostics (Ellenbogen K A, Gunderson B D, Stromberg K D, Swerdlow C D. Performance of Lead Integrity Alert to assist in the clinical diagnosis of implantable cardioverter defibrillator lead failures: analysis of different implantable cardioverter defibrillator leads. Circ Arrhythm Electrophysiol. 2013; 6:1169-1177).
Evaluation of false positive diagnostics adds cost and work to medical care and may contribute to patient anxiety. If a false-positive diagnostic is not diagnosed correctly, patients may be subject to unnecessary surgical lead replacement with its corresponding risks, and clinical reports document that this has happened (Swerdlow C D, Sachanandani H, Gunderson B D, Ousdigian K T, Hjelle M, Ellenbogen K A. Preventing overdiagnosis of implantable cardioverter-defibrillator lead fractures using device diagnostics. J Am Coll Cardiol. 2011; 57:2330-2339).
Gunderson et al. (U.S. Pat. No. 7,369,893) further describes a method for withholding delivery of ICD shocks if ventricular fibrillation is detected from analysis of the pace-sense lead, but not confirmed by analysis of the high-voltage lead. The presumption is that these signals do not represent true cardiac activations. However, this method requires sufficient oversensing of spontaneously-generated, unpredictable, rapid noncardiac signals to cause inappropriate detection of ventricular fibrillation clinically. Thus, it does not provide early diagnosis of conductor anomalies. Further, withholding shocks for ventricular fibrillation detected on the near-field electrogram has an inherent risk of withholding life-saving therapy, however small, if a false positive test outcome occurs. It is thus not the preferred approach to diagnosis conductor fracture. St. Jude Medical has also introduced an algorithm (“SecureSense®”) that incorporates features similar to those described in U.S. Pat. No. 7,369,893.
Gunderson (U.S. Patent Publication No. 2011/0054558) also disclosed applying a pacing stimulus through a pace-sense channel and monitoring the same sensing channel for the occurrence of rapid, anomalous signals immediately after the pacing pulse. This method has the potential to detect anomalous signals when they are not occurring spontaneously, but it has several limitations. First, only conductor fractures are known to exhibit this behavior of pacing-induced lead “noise.” Insulation breaches have not been reported to cause pacing-induced lead noise. Thus, this method does not apply to them. Second, pacing induced lead noise is inconsistent. It does not happen every time a pacing pulse or train is delivered. The infrequency with which pacing-induced lead noise is identified clinically and the infrequency of reports in the medical literature suggests it is uncommon. Thus this method is likely to be insensitive. Third, it does not apply to failures of high-voltage cables or coils.
Each of these algorithms identifies lead failures using abnormal signals on the sensing channel. Thus, they cannot identify failures of high-voltage components including the shock coils and their cables, and they cannot discriminate such signals from other rapid, oversensed signals. What is desired is a method to provide sensitive and specific diagnosis of lead anomalies at the subclinical stage, a method that applies to both pace-sense and shock components.